Diquarks and antiquarks in exotics: A menage a trois and a menage a quatre

摘要

A menage A trois is very different from an ordinary family. Similarly, exotic hadrons with both qq and qq pairs have important color-space correlations that are completely absent in ordinary mesons and baryons. The presence of both types of pairs requires attention to the basic QCD physics that the q (q) over bar interaction is much stronger than the qq interaction. This new physics in multiquark systems produces color structures totally different from those of normal hadrons, for example the ud system is utterly unlike the ud diquark in the uds Lambda baryon. The colorspace correlations produce unusual experimental properties in tetraquarks with heavy quark pairs which may be relevant for newly discovered mesons like the X(3872) resonance. Tetraquark masses can be below the two-meson threshold for sufficiently high quark masses. A simple model calculation shows the bq (b) over bar(u) over bar and bq (c) over bar(q) over bar tetraquarks below the B (B) over bar and B (D) over bar thresholds. Some of these states have exotic electric charge and their decays might have striking signatures involving monoenergetic photons and/or pions. (c) 2006 Elsevier B.V. All rights reserved.